Workers in the field of high-speed document processing (e.g. the sorting of bank checks and like financial instruments), know that the art involves using of machines and systems to move and, en route, process documents at sustained rates of thousands of documents per minute, while performing multiple and inter-related operations upon each document as it travels. Such operations might include (but are not limited to) printing on a document (e.g. by contact, dot-matrix, or ink-jet means), reading data previously encoded thereon, recording an archival image of the document by photographic or electronic imaging processes.
Workers will further understand that documents moving through such machines must be very accurately positioned in order to process them accurately. For example, if a document carries a previously encoded line of magnetic characters (known in the art as MICR characters) and the machine is equipped to read and decode such characters, then the machine must present the document to such reading system in correct alignment and position. Other machine systems have similar requirements for correct and reliable alignment and positioning of documents passing through.
Workers will further understand that document-processing now mandates the user of continuous-feed machines, in which the documents are fed in a continuous stream, and at very high speed; with only negligible gaps between them--otherwise the processing rates presently required by customers could not be achieved. As an example, the Unisys DP1800 is a high-speed document processing machine which, will feed 1800 standard documents per minute at a nominal track speed of 300 inches per second (ips) (7.62 meters per second), with an inter-document gap of as little as 3 inches (75 millimeters) which corresponds, given nominal track speed to an inter-document interval of a mere one-hundredth of a second (10 milliseconds).
Workers will understand that such brief inter-document intervals do not allow a document to be individually aligned and adjusted relative to any process station; rather, it must be kept properly aligned as it passes the stations at all times. Even in "slower" machines (e.g. the Unisys DP500, with a nominal track speed of 100 ips or (2.54 meters per second) the interval between documents is still too brief to permit individual alignment.
Workers will also understand that such document speeds and feed rates requires special transport means: typically high-speed special transport friction-drive rollers and belts, serially arranged to drive the document from point to point. The documents are typically trapped between opposed sets of such rollers and belts, and constrained from distortion by flanking walls. Various process stations are then mounted along said walls, so the walls must be constructed and adapted to permit said process stations to operate upon the passing documents. The need for such transport means and associated track walls is essentially independent of the speed of the document; e.g. they are required in machines of "moderate" speed (e.g. the Unisys DP30, with a nominal track speed of 15 ips (or 0.38 meters per second) as they are in such high-speed machines such as the Unisys DP1800.
Machines here contemplated are designed along these lines, with one convenient construction characterized by a narrow vertical track channel, with walls whose height approaches that of the documents (vertical height); e.g. the order of 4 inches (100 millimeters). Such track walls typically include apertures of various shapes and sizes to permit the driving means (rollers, belts and so on) to drivingly-contact documents in the track. While the track is preferably straight for ease of construction and best document flow, it may also include turns and corners. One finds that good, consistent feeding and driving of documents is obtained if the channel has a very small lateral width relative to its height. Optimal track width will usually vary with document speed. As an example, the nominal track width of the DP1800 product, with a track speed of 300 ips, is 0.10 inches (2.5 millimeters). At lower speeds, narrower track widths may be employed, with consequent improvement in document handling and alignment. As speeds increase, the track width must be increased somewhat, since the increased kinetic energy of the document makes it more likely to jam if the track is too narrow. Workers will readily understand the "trade-off" which must be made between optimal document alignment and consistent document feeding and transport.